What Free 2D CAD programs can export to Blender?
I understand anything that will export/save 'SVG' files. SVG can be imported into Blender. There's quite a few questions about how Blender treats SVG in here.

An SVG option under save/export has rapidly become fairly standard for most vector drafting applications. I'm not up on the free stuff right now, so I'm afraid you'll need to do the leg work to find out which one's will save SVG files. Hope this helps.